Wastewater generated in industrial, agricultural, and other processes often contains unwanted oily droplets, mineral particulates, cellular debris, crop residuals, and other solid and semi-solid matter. Before the water is re-used, recycled to the process, or released to the environment, it is preferably restored to a solids-free, clear condition. Prominent examples include removal of oil and oily solids in water used in oil recovery operations. Similarly, soil components and biological remnants may occur in the water used in recovery and handling of crops and in food processing.
In the case of oil recovery operations, water is used for a variety of reasons. For example, it may be injected as superheated steam into the formation to facilitate liquefaction of the oily deposits at depth followed by movement of the pressurized, oily aqueous stream to the surface. Here, the oil is separated from the water. Typical ratios of water to oil in such operations range around 2 to 3.0. That is, there often results approximately 2 to 3 barrels of water containing oily residuals per barrel of oil that is recovered.
In principal, the oil may be separated from the water via straightforward separation techniques, such as flotation and skimming, that take advantage of the differences in the densities of oil and water. However, in practice, the leftover water stream after surface skimming or raking, decanting, or preferential draining is often dark and oily, due to the presence of stable emulsions and suspensions of oil and oily solids in the water.
These components are thought to be ionically dispersed in the water as oily micelles having outwardly facing anionic groups such as carboxylates and, to a lesser extent, sulfates, sulfonates, phosphates, and phosphonates that are covalently linked to aliphatic, cyclic, and heterocyclic hydrocarbon moieties of complex and heterogeneous composition. In addition to the oily droplets emulsified in the water column, there can be significant components of mineral residuals such as micron-scale particles of sand and clays. Total suspended solids and oils can range upwards of 30% by weight and higher. Even in the range of 1% by weight or less, they can render the water unusable without further clarification and separation steps.
Ideally, in zero discharge approaches to oil recovery such as steam-assisted gravity drainage (SAGD) or cyclic steam stimulation (CSS), the water needs to be clarified to acceptable levels so that it can be recycled to the steam generators without fouling and clogging the boilers or downstream process equipment such as organic removal filters and ion exchangers. In larger-scale, open-mining operations, ultimately it is desirable to return the clarified and cleansed water to the environment, typically a river, from which it originated.
As exemplified in SAGD and CSS operations, the oil is recovered from an aqueous process stream that is brought to the surface from the depths of the formation, still superheated and under pressure. This process water typically involves volumes of 1000's of gallons per minute flowing at rates of 8 feet per second or higher. The flow often occurs through pipes of 10 inches diameter or higher throughout many of the steps of the process. However, the stream is slowed down at certain steps that do not tolerate rapid flow and high shear conditions. For example, shortly after reaching the surface, the stream is directed into very large vessels such as high-temperature separators where the bulk of the oil and water naturally and largely separate mainly based on their densities, invariably with some chemical enhancements such as added de-emulsifiers to promote both rate and degree of separation. The oil is removed and sent to pipelines and tankers for transport to upgraders and refineries for further processing.
Removal of the bulk of recovered oil leaves a reverse emulsion of residual oil and oily solids in water, termed produced water. Roughly three barrels of oily and bituminous-containing water are typically produced per barrel of oil. The solids content of produced water at this stage is typically 1-2%, and at later stages (e.g. slop streams or tailings ponds) it can range from 1% to 60% solids, often in the range of 30% to 60% solids. Produced waters from surface mining can also have high solids contents, e.g. in the 30% range.
The produced water obtained after removal of the bulk of recovered oil is cooled via heat exchangers to a temperature in the range of 90° C., so that it can be handled more readily in the subsequent downstream process steps that include addition of water clarification chemicals. Because of the high oily and bituminous content of the produced waters, and the elevated temperatures involved, it has been challenging to design effective water-treatment protocols that clarify the water and provide good separation of the aqueous and petrochemical phases.
A coagulant is first added to the produced water to disrupt the anionic dispersion and promote coalescence of oily droplets and solids into small particulates. At this point, in SAGD operations, the produced water stream so treated is transferred, typically at high velocity (e.g. about 8 feet/second), into large skim tanks with low velocity and high residence times (typically several hours). Floating oily solids are removed by skimming.
The partially clarified produced water may then be transferred into dissolved gas flotation devices, which generate microbubbles by introducing a solution prepared by dissolving a gas at high pressure and releasing the pressure such that the gas is released from solution. The bubbles so generated are intended to stick to the oily particulates and make them float, and the resulting float is decanted so that the produced water stream can be further clarified.
The stream is then directed through organic removal filters so that any residual potential foulants will not reach the final step of ion exchange prior to return of the water to the steam generator and re-injection into the well. The ion exchange membranes and devices do not tolerate oil and oily residuals and can be easily ruined by an input of improperly de-oiled water, either suddenly or gradually. This results in downtime, very costly both in lost production and replacement of ion exchangers as well as other components of the de-oiling line.
Conventional treatments of oily produced water streams, as described above, are often ineffective, resulting in only partially clarified water. Even if the treatment does result in coalescence of oily solids and partitioning of oily and aqueous phases, the subsequent attempts at removal of residual oil and oily solids are often inefficient.
The conventional processing steps for clarification of produced water are designed to allow skimming of the oil and oily solids from the upper levels and surface of separators, skim tanks, and flotation devices. However, the density of the oily solids and even the oil itself is often very close to that of the water itself. Consequently, the materials that are targeted for removal (the oil and oily solids) either do not float in the first place, or they become readily dispersed throughout the water column, even with only minimal turbulence.
In addition, the microbubbles that are introduced in the flotation devices for the purpose of attachment to the oily solids, as described above, either do not attach or are too poorly attached to survive the turbulent conditions of the flotation devices, making the flotation step unacceptably inefficient. Hence, the downstream produced water stream very often is overly oily and contaminated with particulates, resulting invariably in costly repairs and downtime.